From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, April 26, 2013
Link to streaming video: available 04.26.2013 at 8:55am
Lesli J. Wood
Quantitative Clastics Laboratory
Bureau of Economic Geology, Jackson School of Geoscience
The University of Texas at Austin
Formation of the Gulf of Mexico basin initiated in the Jurassic as product of the rifting of the North American continent from the South America continent. This early rift basin immediately began a fill history that continues today, a history which has become the target of exhaustive geoscientific study and which constitutes the reservoirs and fluids that form the cornerstone of today's U.S. energy endowment. However, our investigation of this immense basin has been a top down effort, with the upper 12,000 feet of strata the focus of decades of effort, while the deeper pre-Neogene strata have suffered from neglect. No more. We now have a growing realization that billions of barrels of oil and trillions of cubic feet of gas may lie, waiting to be harvested in Paleogene and Mesozoic-age reservoirs in the deep Gulf of Mexico. Technological advances in imaging and drilling, new data and ongoing discovery thinking are pushing us to access what lies beneath the traditional hydrocarbon basement. The future of the Gulf of Mexico basin lies in its older past, most specifically the Paleogene and Cretaceous strata.
This talk will focus on the Cretaceous strata and prospectivity in the Gulf of Mexico. Source-to-sink, the late Cretaceous systems of the Gulf of Mexico basin are clastic. Onshore these systems deposited hundreds of feet of coarse-grained fluvial and deltaic facies developing in an overall eustatically transgressive shoreline setting. As proof positive that sediment can overwhelm eustacy, the Late Cretaceous shelf edge deltas of the Tuscaloosa/Woodbine (T/W) units fed significant volumes of high density turbidites basinward, in to repositories hundreds of kilometers from the shelf edge. Well penetrations of the upper Cretaceous in the Perdido Fold belt area of the southwestern GOM show T/W turbidites cycles occurring in bathyal to abyssal settings, stacking hundreds of feet of moderate net:gross sands and mudstones in several sharp-based, fining-up packages. More recently released data from the Tiber well in the Keathley Canyon area of the GOM shows the T/W interval to be composed of poorly sorted, high net:gross, high density turbidites with no more than 11% porosities and dismal permeabilities (less than a md in most cases). Quartz overgrowth is pervasive, uninhibited by chlorites and other clays which can occlude such pore destruction in shallower marine and fluvial T/W facies. Nevertheless, these sands are enormously thick, stacked in several cycles of 100-150 feet thick, blocky deposits. Overlying salt thicknesses act to inhibit high thermal maturation and sands are oil saturated at ~30,000 feet. Seismic windows imaging the Cretaceous in the Mad Dog field of the southwest Atwater Fold Belt show the Cretaceous interval consisting of approximately 1500 m wide amalgamated channel complexes, composed of high seismic amplitude, low sinuosity, ~200-400 m wide individual stacked channels. These channel systems are accumulating within accommodation space troughs that trend northwest to southeast and whose occurrence is most likely tied to the underlying trend of basement transfer faulting. This overall northwest to southeast trend is complicated by the occurrence of salt pillows which are themselves likely influenced by varying thicknesses of original salt in the underlying configuration of basement blocks.
As many as three feeder systems for these Late Cretaceous turbidite reservoirs may exist. The easternmost deposits are sourced from a series of large (30-80 km wide) valleys draining north-northeast to south-southwest out of the ancestral Appalachians and culminating in large estuaries and point-sourced shelf edge deltaic complexes centered in the Alabama-Florida panhandles, the Mississippi Panhandle and the western Mississippi/Louisiana panhandle. The central northern GOM systems are likely sourced from a similar series of drainages trending southward from the eastern Ouachita Mountains, through the Louisiana salt basin in to depo-sites along a wave-dominated strandplain between the Sabine uplift and the LaSalle Arch. Sources for deposits in the Perdido fold belt are a bit less well constrained at this time, but are likely coming from early Laramide drainages through northern Mexico.
The late Cretaceous depositional system of the GOM is but one highly prospective phase of fill within the deep basin that bears examination. Oil saturation at 30,000 feet attests to the power of the salt canopy to affect normal thermal gradients in the GOM. Additional understanding of topography on the Cretaceous accommodation surface and of the behavior of high density turbidites in this topography will lead to additional insights into these system that reside beneath in the deep strata of the GOM.